A light emitting diode (LED) is a solid state device that converts electrical energy to light. Light is emitted from active layers of semiconductor material sandwiched between oppositely doped layers when a voltage is applied across the doped layers. The active layers typically include InGaN materials that emit blue light. In order to produce white light, wavelength converting materials commonly known as phosphors are used to convert part of the blue light emitted by the LED into light of longer wavelengths, such as yellow and red, so as to generate a combination of light having the desired spectral characteristics. Examples of phosphors are yttrium aluminum garnet (Y3Al5O12 or YAG) and terbium aluminum garnet (Tb3Al5O12 or TAG). The combination of blue and yellow light appears to the human eye as “cool white,” which has a color temperature in the range of 5000-8000 degrees Kelvin, corresponding to daylight. By adding red to the blue and yellow light, “warm white” light can be generated with a color temperature in the range of 2700-3500 degrees Kelvin, which is used for indoor lighting.
FIG. 1 (prior art) shows a conventional LED device 10 in which the phosphor 11 is deposited as a layer over an LED die 12 disposed on the upper surface of a substrate 13. A silicone lens 14 is molded over LED die 12 and phosphor plate 11. Phosphor plate 11 covers only the top surface of LED die 12 and does not cover the sides. The light that is emitted from lens 14 contains different amounts of blue and yellow at different angles. For example, the light that is emitted orthogonally from the top surface of LED die 12 and out through the top of the domed lens 14 (at zero degrees) is much bluer than the light that is emitted in a ring at about seventy-five degrees from orthogonal.
FIG. 2 (prior art) is a graph of the angular color performance of LED device 10 of FIG. 1. The graph shows how the color of the light emitted from lens 14 changes over the angle of the emitted light. The colors are defined by the color coordinates x and y of the CIE XYZ color space created by the International Commission on Illumination (CIE). The dashed line 15 indicates the change in the color coordinate x (Δ ccx) over the emission angles −90° through 0° to 90°, as those angles are shown in FIG. 1. The solid line 16 indicates the change in the color coordinate y (Δ ccy) over the same range of emission angles. Solid line 16 shows that the value of the color coordinate y increases by as much as 0.044 over the range of emission angles from 0° to 75°, while the color coordinate x also increases over the same range. Thus, LED device 10 produces a bluish spot centered at 0° and a yellowish ring at about 75°. Customers of LED device 10 find the bluish spot and yellowish ring to be undesirable and would prefer a uniform white light to be emitted at all angles.
FIG. 3 shows an LED device 17 that exhibits a somewhat improved angular color performance compared to LED device 10 of FIG. 1. The phosphor in LED device 17 is disbursed throughout the silicone that forms an hemispherical lens 18. The silicone lens 18 is molded onto the upper surface of a substrate 19 and over an array of LED dies that includes die 20 and die 21. But LED device 17 also exhibits some non-uniformity in its color-over-angle light emission characteristics.
FIG. 4 is a graph showing the angular color performance of LED device 17 of FIG. 3. The graph shows how the color of the light emitted from lens 14 changes over the angle of the emitted light. The colors are indicated with the color coordinates x and y of the CIE-1931 XYZ color space. The dashed line 22 indicates how the color coordinate x (ccx) changes over the emission angles −90° through 0° to 90°, as those angles are shown in FIG. 3. The solid line 23 indicates the how the color coordinate y (ccy) changes over the emission angles −90° through 0° to 90°. FIG. 4 shows that LED device 17 produces a bluish ring at about ±55° between a yellowish spot at 0° and a yellowish ring towards ±90°. The color performance indicated in FIG. 4 applies to LED dies and phosphors that produce warm white light corresponding to about 3000° K. Where less phosphor is used to produce cool white light, such as corresponding to 5600° K, the variation in the color over the range of emission angles is even larger.
FIG. 5 shows the change in values of the color coordinates x and y indicated in FIG. 4. The change in the values of the color coordinate x over the range of emission angles is about the same as the change in the values of the color coordinate y, so dashed line 24 represents the change in each of the color coordinates x and y. The values of the color coordinates x and y increase by as much as 0.04 from about 60° to 90° and from about −50° to −90°. Although the change in color over emission angle for LED device 17 is somewhat less than for LED device 10, the bluish ring at about ±55° from LED device 17 is more noticeable than the bluish spot at 0° from LED device 10. The bluish spot at 0° is less perceptible because the color is constantly becoming more bluish as the emission angle decreases for LED device 10, whereas the color emitted by LED device 17 becomes more bluish towards the ring at about ±55° and then rapidly becomes more yellowish. The human eye is more sensitive to the change toward more blue and then less blue than to a constant change in one color direction. Customers of LED device 17 find the bluish ring to be undesirable.
An LED device is sought that emits light with a more uniform color distribution over the entire range of emission angles so that colored spots and rings are less perceptible.